Boronic acid modified polyoxometalate-alginate hybrid for the isolation of glycoproteins at neutral environment

Boronate affinity is widely used for the isolation of glycoproteins at alkaline conditions. For further proteomic studies, however, it is of highly importance to perform protein adsorption in neutral medium. For this purpose, we report a novel composite material, i.e., 4-carboxyphenylboronic acid (CPBA) functionalized nickel-substituted polyoxometalate [Ni₆(en)₃(Tris)(H₂O)₂(PW₉O₃₄)]•nH₂O (Ni₆PW₉)-sodium alginate (SA) hybrid. The abundant oxygen atoms in the hybrid (shortly termed as CPBA-Ni₆PW₉/SA) significantly reduce the pKa value of CPBA moiety, which well facilitates the selective adsorption of glycoproteins at neutral environment (at pH 7.0). The moiety of sodium alginate (SA) in the hybrid further improves the isolation/enrichment capacity for glycoproteins through hydrophilic interaction. The adsorption efficiency of Immunoglobulin G (IgG, 1.0 mL, 100 μg mL^-1) by 1.0 mg CPBA-Ni₆PW₉/SA hybrid reaches up to 91%, and 1.0 mL of Tris-HCl buffer (100 mmol L^-1) provides an elution efficiency of 82%. The adsorption behavior of IgG fits Langmuir adsorption model, offering a maximum adsorption capacity of 495 mg g^-1. The CPBA-Ni₆PW₉/SA hybrid is practically applied for the enrichment of glycoproteins from human serum. SDS-PAGE assay result indicates that approximately 92% serum albumin is eliminated and high-purity IgG is obtained. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis clearly demonstrated that after enriching with CPBA-Ni₆PW₉/SA, 81 glycoproteins are identified and 79.4% recognition selectivity is achieved.